In the coming year, work will proceed along two main lines. The study to determine the mechanism of muscle contraction will concentrate upon separating sarcomere motion from motion at the ends of the muscle, the former being greatly influenced by tendon compliance. In studying this we will make use of optical diffraction techniques. Stiffness measurements based upon the speed of propagation of small mechanical disturbances, will be used as a measure of cross-bridge interaction during isotonic shortening. Work on the study of excitability in cardiac Purkinje fibers will be directed towards elucidating the effect of ionic currents generated in clefts of the fiber upon excitation.